1. Field of the Invention
The present invention relates to a cathode electrode, to a method of manufacturing the cathode electrode, and to a lithium sulfur battery using the same. More particularly, the present invention relates to a cathode electrode in which the amount of a binder used in the cathode active material composition, which also includes sulfur or polysulfide as the cathode active material, is reduced. The invention also relates to a method of manufacturing such a cathode, and to a lithium sulfur battery using the same.
2. Description of the Related Art
With the increasing demand in recent times for miniaturized, lightweight portable electronic devices such as cellular phones, notebook type computers, camcorders and the like, there is a concomitant increase in demand for a lithium secondary battery as power sources for such devices, whereby the battery can realize a smaller size, is lightweight, and has a large capacity. Various materials can be used as active materials for secondary batteries, and conventional lithium ion batteries and lithium polymer batteries typically use a lithium metal compound, e.g., LiCoO2, as a cathode active material, and either crystalline or non-crystalline carbon as an anode active material.
The theoretical capacity of LiCoO2 as a cathode active material is only 274 mAh/g, and the theoretical capacity of carbon as an anode active material is only 372 mAh/g. Although batteries have been smaller and lighter along with the advancement of battery manufacturing technologies, realization of small, lightweight batteries typically is defined by theoretical capacities of cathode and anode active materials.
Lithium sulfur secondary batteries that use sulfur as a cathode active material, on the other hand, have a theoretical capacity of 1680 mAh/g for the cathode, and a lithium metal having a theoretical capacity of 3860 mAh/g as an anode active material. These batteries have a very large energy density, when compared to conventional lithium ion batteries and lithium polymer batteries, and have a potential in manufacturing small, lightweight batteries that are in increasing demand today.
In lithium sulfur batteries, the oxidation/reduction reaction between lithium and sulfur can be expressed by the following reaction scheme. It is known that the reaction capacity of sulfur that can be practically used in a lithium secondary battery is only a half a theoretical capacity, that is, approximately 840 mAh/g due to irreversible reaction characteristics of polysulfide.
2Li+S8 (solid)⇄Li2S8 (solution)
2Li+Li2S8 (solution)⇄2Li2S4 (solution)
2Li+Li2S4 (solution)⇄2Li2S2 (solution)
2Li+Li2S2 (solution)⇄2Li2S (solid precipitate)
As seen from the above reaction schemes, in the oxidation/reduction reaction between sulfur and lithium, a new reaction product, that is, lithium polysulfide, is generated. It is known that sulfur and lithium polysulfide participating in the above reactions have very low electrical conductivity. In order to promote an electrochemical reaction, it is necessary for active materials to contact the surface of a conductive reaction site. Also, in order to promote the supply of electrochemical reaction sites, it is necessary to obtain a sufficient reaction surface area by using a large amount of a conductive agent.
In conventional lithium ion batteries and lithium polymer batteries, the oxidation/reduction reaction is an intercalation reaction in which lithium ions move into/from a laminate structure of LiCoO2. In such an intercalation reaction, since the electric conductivity of LiCoO2 is still low, a conductive agent is required to increase the conductivity of the cathode. Since the cathode active material LiCoO2 acts as a reaction site, only a minimum amount of a conductive agent required for increasing the conductivity of a cathode electrode, is necessary.
Accordingly, a larger amount of a conductive agent is used for a cathode in a lithium sulfur battery, when compared to a lithium ion battery or lithium polymer battery. For example, U.S. Pat. No. 5,961,672-4, WO 33125-3, WO 33125-4, WO 33127-2 and WO 33127-3, the disclosures of which are incorporated by reference herein in their entirety, describe that as much as 10 to 30% by weight of a conductive agent is used.
Increasing the amount of a conductive agent unavoidably involves an increase in the amount of binder used so as to prevent bondability of a cathode plate from lowering. This results in a reduction of the concentration of a cathode active material in a cathode active material layer including a conductive agent and a binder in addition to the cathode active material, which may, in turn, become an impediment to the manufacture of high-performance cathode electrodes.
The description herein of certain disadvantages of the known batteries, methods, and apparatus is in no way intended to limit the scope of the invention. Indeed, certain aspects of the present invention may include various features of the known batteries, methods, and apparatus without suffering from the disadvantages described herein.